1. Field of the Invention
The present invention relates to an ink composition comprising an infrared ray-emitting fluorescent substance which is excited by infrared ray and emits light in the infrared region, and printed material which is printed using said ink composition. In particular, the present invention relates to an ink composition in which an infrared ray-emitting fluorescent substance is well dispersed having excellent light-emitting characteristics, and the printed material on which the infrared ray-emitting layer with excellent light-emitting characteristics is printed using the ink composition.
2. Description of the Related Art
To prevent forgery and maintain secrecy, an infrared ray-emitting layer which cannot be visible to the naked eye such as a stealth bar code is printed on a printed material such as a brochure or catalogue using an ink composition comprising an infrared ray emitting fluorescent substance which emits light in the infrared region, and the code information is read by an apparatus for optically reading information of goods such as properties and a price or various personal information (see Japanese Patent Publication No. 18231/1986 and Japanese Patent KOKAI Publication No. 9600/1978). It has been attempted to print an infrared ray-emitting layer on a plastic substrate, such as a prepaid card, an ID card and a magnetic card, or a magnetic layer using such ink ribbon.
As an infrared ray-emitting fluorescent substance to be used in a conventional ink composition which is printed to form an infrared ray-emitting layer, for example, a phosphoric acid base infrared ray-emitting fluorescent substance of the formula:ps EQU QD.sub.1-x-y Nd.sub.x Yb.sub.y P.sub.4 O.sub.12
wherein Q is at least one element selected from the group consisting of Li, Na, K, Rb and Cs; D is at least one element selected from the group consisting of Sc, Y, La, Ce, Gd, Lu, Ga and In; x is a number of 0.05 to 0.999 and y is a number of 0.001 to 0.95, provided that a sum of x and y (x+y) is 1.0 or less is disclosed in Japanese Patent Publication No. 22326/1979 and widely used.
However, the conventionally used infrared ray-emitting fluorescent substance has poor dispersibility and may not provide an ink composition having a sufficient emitted light intensity. In particular, since the phosphoric acid base infrared ray-emitting fluorescent substance of the above general formula has a large particle size of at least 7 .mu.m, it should be ground to a smaller particle size when a UV curable resin is used as a binder resin, or when the ink composition comprising the fluorescent substance is printed by off-set printing or by ink jet printer. But, by grinding, crystallinity or composition of the infrared ray-emitting fluorescent substance is deteriorated, its dispersibility is greatly deteriorated, an intensity of emitted light is greatly decreased and duration of afterglow is shortened.
To improve the dispersibility of the conventional infrared ray-emitting fluorescent substance in the ink composition, there is used a surfactant, such as alpha-olefinsulfonic acid salts, alkylbenzenesulfonic acid salts and alkylphenols, as a dispersant. But the dispersibility is not sufficiently improved by such a dispersant.